1. Technical Field
The present invention relates generally to very sensitive thermometric instruments, known as microbolometers, which are used for the detection and measurement of radiant energy. More specifically, the present invention addresses correction of microbolometer output.
2. Related Art
Infrared detectors known as microbolometers respond to impinging infrared radiation through subtle variations in the temperature of the detector element. The detector elements include a material having a high temperature coefficient of resistance (TCR) such that these subtle variations in the temperature of the detector may be sensed. The sensing methods often employed are based on the passing of a metered electrical current through the device and measuring the resulting voltage drop. Alternatively, the temperature of the detector may be sensed by biasing the detector circuit with a known voltage and measuring the resulting current. In the simplest embodiment, the microbolometer detector is connected to a meter, and the response of the meter can be correlated to the intensity of the impinging infrared radiation.
However, in typical applications for which an image is desired, a lens is employed to focus energy onto a two-dimensional array of microbolometer detectors such that a spatially varying infrared field can be detected and converted to visible imagery using electronics and display means such as are commonly employed for visible imagery using Charge-Coupled Device (CCD) cameras. The electronics typically include a multiplexing circuit in intimate contact with the microbolometer array which converts the voltage or current variation of the many microbolometer elements to one or several multiplexed analog (e.g., voltage variation) data streams. This analog data is then converted to digital data using an analog-to-digital converter (ADC), and is then further processed to produce data for analysis or imagery on a Cathode Ray Tube (CRT) or similar video monitor.
The fact that the detection means is based on the thermal variations of the detector causes several practical problems. First, the material must be thermally isolated from surrounding matter so that a sufficiently large (e.g., several mK) temperature variation may occur as a result of the weak impinging infrared energy. Liddiard, in U.S. Pat. Nos. 4,574,263 and 5,369,280, and Higashi, et al., in U.S. Pat. No. 5,300,915 describe a microbolometer that provides thermal isolation by depositing a semiconductor material onto a pellicle, or xe2x80x9cmicro-bridgexe2x80x9d structure that physically separates the detector from the supporting substrate. Second, the temperature of the supporting substrate must be stable so that erroneous signals are not generated from its temperature fluctuations. Experience indicates that a 15 mK variation of substrate temperature within the sampling period (or video frame rate, whichever is greater) is acceptable, but fluctuations greater than this present a significant source of system noise. Third, the output of the microbolometer varies as a result of both the impinging infrared radiation, and the absolute temperature of the substrate. In this last case, the array output may be higher or lower at different temperatures, even if that temperature is held to within the stability requirement of 15 mK. Fourth, variations in the physical construction of the microbolometer detectors result in significant variations of the output of individual microbolometer detectors within the array, and these non-uniformities must be corrected in order to obtain a low-noise image.
As a result, there exists a need for an apparatus capable of correcting the output of a microbolometer, for example, in a focal plane array (FPA), such that the effects of thermal drift are removed or eliminated.
In the particular problem of thermal variation of the substrate, microbolometer detectors are operated at a fixed temperature, typically with a stability tolerance of xc2x10.015xc2x0 C. (i.e., 15 mK). Peltier-junction heat engines and control circuitry are commonly employed for this purpose. While this temperature stabilization scheme works well, it is not the ideal solution. For instance, the temperature stabilization system represents a significant portion of the detector package cost. Further, it is susceptible to damage from shock or vibration, and ordinarily requires tens of seconds to reach operational temperature from system start-up. Also, the temperature stabilization means is a major consumer of system power.
Since the output of the microbolometer varies as a result of impinging infrared radiation, a number of additional noise sources and undesirable effects occur. Referring now to FIG. 7, the microbolometer is impinged by infrared radiation from the cold shield, 608a, the lens, 616a, the dewar window, 606a, as well as the signal 622a. If the temperature of the cold shield 608a, lens 616a and dewar window 606a remain constant, then variation in their radiant flux also remain constant. The microbolometer output voltage or current due to variations in the signal emanating from source 618a may then be determined by subtracting the fixed voltage or current offset arising from the impinging radiation from the cold shield, lens and dewar window. If, however, the temperature of the cold shield 608a, lens 616a and dewar window 606a vary more than a few degrees Celsius, a significant source of uncertainty in the microbolometer output, termed noise, is created.
Due to limitations in the input sensitivity range of the analog-to-digital converter (ADC), these noise sources may cause under or over saturation of the ADC, resulting in the loss of sensitivity to desired signal data. The radiant flux from the source may also vary more than the input sensitivity of the ADC permits, and loss of sensitivity to desired signal data may also result.
In the particular problem of modulating the detector""s sensitivity to widely varying radiant flux from the source, several methods are typically employed to maximize dynamic range. The most common method is to insert a mechanical aperture stop within the system optical path to vignette energy and effectively change the focal ratio (or xe2x80x9cfxe2x80x9d number). However, this method requires mechanical components that are bulky, expensive and unreliable, and require the user to manually adjust the sensitivity. A better method is to vary the duty cycle of the detector by means of changing the time that the sensor is being sampled by the system electronics. This method is a distinct improvement over the manual method, but has the disadvantage of requiring a new calibration each time the sampling time is changed. While the calibrations can be stored in digital memory, the quantity of memory required and the number of calibrations that must be performed tend to increase system size and cost.
In the particular problem of correcting for thermal variations of the cold shield 608a, lens 616a and dewar window 606a, the most common method is to perform frequent system calibrations to eliminate these effects. Many thermal imaging systems have built-in motorized calibration sources for this purpose. However, calibrating the system frequently reduces the availability of the system for its intended use and consumes system power. Further, the motorized calibration source (shown as 630 and 631 in FIG. 6) decreases the reliability of the system due to its moving parts and increases manufacturing costs.
Therefore, there exists a need for an apparatus capable of correcting the output of a microbolometer, for example, in a focal plane array (FPA), such that the effects of thermal variations in the cold shield 608a, lens 616a and dewar window 606a are reduced or eliminated. This apparatus should also have the ability to modulate the sensitivity of the microbolometer to large variations in the radiant flux (signal) from the source so that the microbolometer output remains within the input sensitivity range of the ADC.
It is an advantage of the present invention to provide a system and method for correction of microbolometer output. For example, the present invention provides a method to eliminate the need for gross temperature stabilization of a microbolometer through the creation of a system that uses electronic means to correct the temperature variation of the microbolometer. An advantage of the present invention is that it eliminates the need for recalibration of a microbolometer appliance, for instance a microbolometer camera, should the temperature of the focal plane array in the camera change from the temperature for which it was calibrated. Further, rapid system readiness is possible since thermal stabilization of the focal plane array is not necessary. Specifically, this invention conditions the multiplexed output of a microbolometer focal plane array so that the peak-to-peak voltage of the analog signal is within the range of an analog-to-digital converter""s input sensitivity at any arbitrary temperature between approximately xe2x88x9210xc2x0 C. and 50xc2x0 C.
A further general aspect of this invention is to provide a method of correcting the output of a microbolometer detector system, said method comprising: providing a microbolometer detector system for operation in an ambient temperature range, said microbolometer detector system further comprising at least one microbolometer detector, and an apparatus for reducing thermal noise; providing a temperature stabilization system for correcting the temperature variation of the microbolometer detector; providing an electronic system for conditioning the output of the microbolometer detector; and applying a correction factor to the output signal of the microbolometer detector.
A second general aspect of the present invention is to provide a method of correcting the output of a microbolometer detector system, said method comprising: providing a microbolometer detector system for operation in an ambient temperature range, said microbolometer detector system further comprising at least one microbolometer detector, and an apparatus for reducing thermal noise; providing a temperature stabilization system for correcting the temperature variation of the microbolometer detector; providing an electronic system for conditioning the output of the microbolometer detector; providing a system for varying the responsivity of the microbolometer; applying a correction factor to the output signal of the microbolometer detector.
A third general aspect of the present invention is to provide an apparatus for correction of a microbolometer detector output, said apparatus comprising: at least one microbolometer detector; a thermal noise reduction system operationally connected to said at least one microbolometer detector; an electrical reference circuit connected to said at least one microbolometer detector; an output from said electrical reference circuit connected to an input of a signal conditioning circuit; and an output from said signal conditioning circuit connected to a display device.
A fourth general aspect of this invention is to provide a method for modulating the sensitivity of a microbolometer by conditioning the multiplexed output of a microbolometer focal plane array so that the peak-to-peak voltage of the analog signal is within the range of an analog-to-digital converter""s input sensitivity at any arbitrary sampling time.
A fifth general aspect of this invention is to provide a method for correcting the output of a microbolometer to reduce or eliminate the effects of variation in the temperature of the cold shield, dewar window and lens.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention.